1. Field of the Invention
The invention relates to synchronization and, more particularly, to synchronization using an interpolation filter to selectively vary the decimation ratio to adjust signal timing.
2. Description of Related Art
The use of modems to transmit digital signals across an analog channel, such as a telephone line, is well known in the art. Modem capabilities and performance have increased dramatically as the digital technology utilized to handle information has exploded with a variety of new applications and with large quantities of content. This technology explosion has resulted in increasing complexity for modems required to handle increasingly complex protocols.
Recent generations of modems utilize different signaling rates at different times or stages during their operation. Typically, a modem utilizes a dedicated processor or controller to carry out the operations required for modem transmission and reception. Software which drives such dedicate processors is often convoluted, containing many branches and jumps. Frequently, the dedicated modem processor is controlled by a sequencer implemented as a finite state machine. The state of the finite state machine changes as samples arrive and are sent in such a way as to implement the modem functionality. Typically, incoming signals from an analog channel are sampled by an analog to digital coder/decoder (codec) and signal samples are processed as they arrive from the codec. This imposes certain demanding real time performance requirements since processing of a given sample must be completed by the time the next sample arrives.
Incoming signal levels to a modem are often adjusted by an automatic gain control (AGC) circuit. However, incoming signals are often subject to a line xe2x80x9chitxe2x80x9d which causes a momentary deviation from the desired gain level which cannot be compensated for by the AGC circuit.
Controllerless modems are also known which run as a separate process on the host which they service. An example of such a controllerless modem is shown in the referenced co-pending application.
Modern computers are processing real time audio in digital form more and more frequently. This audio processing can take the form of, for example, telephone applications, stored audio files, audio files accompanying real time motion images and the like. Often, this processing is ongoing at the same time as modem functions are occurring.
Digital signal processors are also known. These are relatively memory limited devices which are designed for high performance processing of digital signals. They typically operate as an adjunct to the host processor and can be configured to receive and handle processing assignments from the host computer and then return the results either to the host or to a memory location specified by the host. Digital signal processors are now available which handle multiple streams of digital signals.
A variety of techniques are used to adjust the timing of two digital signal streams so that important events from both streams coincide in time. These techniques are called synchronization techniques. Frequently, synchronization is required when undertaking modem applications or digital signal processing applications.
The invention is directed to techniques for estimating the frequency difference between local and remote clocks in a communications system and for correcting those clocks using a discrete Fourier transform (DFT) and extracting spectral components from the DFT as a measure of frequency difference which can be used to correct for differences in clock frequency.